The capacitor for use in electronic devices such as cellular phone and personal computer is demanded to have a large capacitance with a small size. Among these capacitors, a tantalum solid electrolytic capacitor has a large capacitance for the size and also has good performance and therefore, this capacitor is preferably used. In order to increase the capacitance further, studies are made on a solid electrolytic capacitor using a dielectric layer mainly comprising niobium oxide having a higher dielectric constant. This niobium-based solid electrolytic capacitor is constituted such that the anode is niobium monoxide, niobium, an alloy mainly comprising niobium, or a mixture of niobium monoxide with niobium or an alloy mainly comprising niobium, the dielectric layer is a layer formed by electrolytic oxidation of the anode and mainly comprising niobium oxide, and the cathode is a semiconductor. For obtaining good ESR value of the capacitor, it is studied to use a highly electrically conducting organic semiconductor as the semiconductor. However, when a highly electrically conducting organic semiconductor is used for the cathode of a solid electrolytic capacitor and the produced capacitor is packaged on a circuit board or the like, there arises a problem that the leakage current (hereinafter, sometimes simply referred to as “LC”) value of the capacitor greatly increases due to heat of soldering at the packaging.